Capillary insulation

ABSTRACT

A capillary insulation assembly for insulating low temperature liquids consists of a cellular core defining discrete cells and a capillary cover having at least one or more openings per cell attached to the liquid side of the cellular core. A structurally stable liquid-gas interface forms at each of the openings preventing the entry of liquid into the cell and thereby positioning a layer of gas between the liquid and the container wall. A thermal barrier in the form of an insulative layer is interposed between the capillary cover and the contained liquid with the insulative layer being operative to insulate the gasliquid interface from subcooled liquid in the container thereby to maintain the temperature at the interface at the saturation temperature.

[ June 13, 1972 [54] CAPILLARY INSULATION [72] Inventor: John P. Gille,Littleton, Colo.

Martin Marietta Corporation, Friendship lntl. Airport, Md.

[22] Filed: Oct. 16,1970

[2]] Appl.No.: 81,440

[73] Assignee:

[52] [1.8. CI ..62/45, 220/9 LG [51] Int. Cl. F 17c 13/00 [58] Field ofSearch ..220/9 A, 9 B, 9 D, 9 LG, 10, 220/15; 62/45 [56] ReferencesCited UNITED STATES PATENTS 2,676,773 4/1954 Sanz et al. ..220/9 A X2,937,780 5/1960 Beckwith ..220/9 LG 3,365,897 l/l968 Middleton et al..62/45 2,859,895 11/1958 Beckwith ..220/65 2,947,438 8/1960 Clauson..220/15 3,018,018 l/l962 Beckwith.... ..220/63 3,019,937 2/1962Morrison ..220/9 LG X nix 3,150,794 9/ 1964 Schlumberger et al ..220/9LG 3,208,621 9/ 1965 Dawson ...220/9 LG 3,261,087 7/1966 Schlumberger220/9 A X 3,325,037 6/1967 Kohn et al 0220/9 A Primary Examiner-MeyerPerlin Assistant Examiner-Ronald C. Capossela Attorney-Phillip L. DeArment and Gay Chin [57] ABSTRACT tioning a layer of gas between theliquid and the container wall. A thermal barrier in the form of aninsulative layer is interposed between the capillary cover and thecontained liquid with the insulative layer being operative to insulatethe gasliquid interface from subcooled liquid in the container therebyto maintain the temperature at the interface at the saturationtemperature.

12 Claims, 3 Drawing Figures PATENTEDJUH 13 m2 3, 668,880

//VVEN7'0/? JOHN A G/LLE FIG. 2 V2 ATTORNEYS CAPILLARY INSULATION Theinvention described herein was made in the performance of work under aNASA contract and is subject to the provisions of section 305 of theNational Aeronautics and Space Act of 1958, Public Law 85-568 (72 stat.435; 42 U.S.D. 2457).

This invention relates to the field of insulation and, moreparticularly, to an improved insulation for use in the containment oflow temperature or cryogenic liquids.

The need for lightweight, inexpensive, reliable non-vacuum insulationfor the containment of cryogenic liquids has long been recognized. Sucha cryogenic insulation is disclosed in copending application Ser. No.44,678, filed June 9, 1970, and assigned to the assignee of thisinvention, and the disclosure thereof is incorporated herein byreference. This insulation provides a layer of gas between the liquidand the wall of the container with capillary forces being used toseparate the liquid and gas phases. More specifically, theafore-mentioned application discloses a cellular or honeycomb core whichforms discrete and separate cells attached to the container wall and acapillary cover attached to the liquid side of the cellular core. Thecapillary cover has one or more openings per cell. Through surfacetension effects a stable gas interface, which acts as a stretchedmembrane, is formed at each of the openings thereby preventing the entryof liquid into the cell and positioning a layer of gas between theliquid and the container wall.

In addition to the importance of surface tension in the establishment ofthe liquid-gas interface, the thermodynamic state of the liquid at theinterface is also of considerable importance in the proper functioningof the insulation. It may be apparent that a thermodynamic equilibriummust exist across this phase boundary. The pressure is slightly greaterin the gas than in the adjacent liquid because of capillary pressureeffects and the liquid immediately adjacent the interface must besaturated at this gas pressure. Assuming the gas in the cells is thevapor of the contained liquid, it is apparent that the temperature atthe interface corresponds to the boiling temperature of the liquid atthe gas pressure.

If the container is depressurized after equilibrium conditions have beenestablished at the interface, the gas in the cells will bubble from thecells through the capillary openings until a new state of pressureequilibrium is established. In the same manner, if the pressure in thecontainer is increased, additional liquid will enter the cell where itwill vaporize to increase the gas pressure in the cell until a newequilibrium condition is established at the interface.

A particular problem arises in maintaining equilibrium conditions at theinterface if the bulk liquid is subcooled. Thus, if the liquid at theinterface becomes cooler than the saturation temperature, condensationof the gas in the cell will begin to occur allowing liquid to enter thecell. A percolating action will tend to develop with liquid alternatelyentering the cells and evaporating and then bubbling back through thecapillary opening into the liquid. This percolating action will continueas along as the liquid immediately adjacent the capillary opening issubcooled. This percolating action substantially reduces the insulatingeffect of a cell and for practical purposes renders it inoperative.

It is the primary object of this invention to provide capillaryinsulation of the type heretofore described which is effective instoring subcooled liquids over long periods of time.

A subcooled liquid is one which is at a temperature below the saturationtemperature or boiling temperature of the liquid. The boilingtemperature of the liquid, of course, also varies with the pressure ofthe liquid, and the boiling temperature increases as the pressure of theliquid increases and, conversely, reduces as the pressure of the liquidreduces. Moreover, when considering the boiling of liquids in a closedcontainer, it should be realized that the partial pressure of the vaporin contact with the liquid also has an effect on the boiling temperatureof the liquid. As the partial pressure of the vapor increases theboiling temperature of the liquid also increases, and conversely. Thesewell-known thermodynamic facts are important to an understanding of theoperation of the present invention. The important point to realize isthat boiling point of a liquid in a closed container can be controlledby controlling either, or both, the temperature of the liquid or thepartial pressure of the vapor adjacent the liquid. The present inventioncontemplates utilizing either or both of these aspects in order toeffectively solve the afore-mentioned percolating problem which occursin the storing of subcooled liquids.

In accordance with the principles of this invention, there is provided acapillary insulation comprising a cellular core defining a plurality ofdiscrete cells with the core being adapted to be secured to the walls ofa container for storing low temperature liquid. A capillary coverextends across the liquid side of the cells with at least one openingper cell in the cover. The capillary openings in the cover are sodesigned that a structurally stable liquid-gas interface is formed ateach of the openings, preventing the entry of liquid into the cells andthereby positioning a layer of gas between the liquid and the containerwall.

In accordance with one aspect of the present invention, to maintainequilibrium conditions at the interface, and more specifically tomaintain the temperature at the interface at the saturation temperatureof the liquid, a thermal barrier of a porous material is placed in theliquid adjacent to the capillary cover to establish a temperaturedifferential between the temperature of the liquid remote from theinterface and the temperature at the interface. The temperaturedifferential results from the thermal resistance of the insulating layerof the nor mal heat flux entering the bulk liquid through the insulationsystem. This thermal barrier, or interior insulation layer, permitsliquid to pass therethrough but is effective to achieve a temperatureStratification in the bulk liquid so that stable equilibrium conditionsare maintained at the interface, even though the bulk liquid may besubcooled. The thermal barrier or insulation layer is not sealed to thecellular core and is either inherently porous or perforated to permitthe passage of liquid through the barrier to the capillary openings. Thematerial from which the insulation layer is constructed and thethickness of that layer may vary depending on the particular conditionsbut, in general, the layer of insulation must be sufficient to establisha temperature differential across the layer which is equal to thedifference between the boiling and bulk temperatures of the liquid atthe minimum rates of thermal conduction or heat flux at which the systemis to be operated.

Other aspects and features of the invention will be apparent upon acomplete reading of the following description which, together with theattached drawing, discloses the invention. Referring to the drawingswherein like reference numerals indicate like parts in various views:

FIG. 1 is a fragmentary perspective view of an insulation assemblyconstructed in accordance with the principles of this invention;

FIG. 2 is a sectional view taken along line 22 of FIG. 1; and

FIG. 3 is an enlarged view of the capillary opening in FIG. 2.

Referring now more in detail to the drawings where the showings are forthe purposes of illustrating a preferred embodiment only, there isillustrated in FIG. 1 an insulation assembly, indicated generally by thereference numeral 10. The insulation assembly 10 is adapted for longterm storage of subcooled, low temperature boiling point liquids, suchas liquified natural gas or liquid nitrogen, oxygen, hydrogen, etc. Theinsulation assembly 10 is adapted to be secured to a supporting surface12 which normally will correspond to the walls of a suitable tank orcontainer in which the liquid is to be stored.

The insulation assembly 10 comprises a cellular or honeycomb coreindicated at 14 which may be constructed from any lightweight materialwhich is compatible with the liquid being stored and which has a lowthermal conductivity. For example, the core may be constructed frompolyimide, Nomex, nylon or plastic impregnated Kraft paper.

The core 14 comprises a plurality of strips or ribbons 16 which areassembled on edge and secured together at spaced points. Each strip hasan undulating cross-sectional configuration with adjacent stripscooperating to define discrete cells 18 therebetween. The edges of theassembled strips cooperate to define opposed, generally planar surfaceswith one of the surfaces being secured to the tank wall 12.

The cells 18 are substantially closed on the liquid side of the core 14by a capillary cover 20. The cover 20 may be made from a material suchas 1 mil Mylar film or a 1 mil Kapton film. Cover 20 extends across eachof the cells 18 and is secured to the cellular core by suitable means,such as adhesive bonding. The portion of the cover over each cell may bedimpled or concave in configuration, as is disclosed in copendingapplication, Ser. No. 81,400, filed Oct. 16, 1970.

The combination of the cellular core 14, the cover 20 and the surface 12cooperates to define a plurality of separate confined areas or voidscorresponding to the cells 18. The cover 20 includes a plurality ofsmall capillary openings 22 with at least one opening per cell. Theseopenings provide communication between the interior of the cells 18 andthe liquid stored in the container. In each of these confined areas ofcells a gas column is established to insulate the stored liquid from thetank wall 12.

The insulating gas column in each of the cells 18 is established in thefollowing manner. A container having the insulation 20 applied to thewalls 12 thereof will be filled with the liquid to be stored. As theliquid enters the container and contacts the capillary cover 20, aportion of the liquid will pass through the capillary openings 22 intothe cells 18. The liquid in the cells 18 will vaporize and build up agas pressure in each cell until the pressure of the gas columns in thecells equalizes with the pressure of the liquid. When this condition isreached, a liquid gas interface 24 forms at each of the openings 22 withliquid on one side of the interface and gas on the other side of theinterface. Because of the small size of the openings and thecharacteristic of surface tension, this liquid-gas interface, underequilibrium conditions, is structurally stable and acts as a stretchedmembrane. While the membrane or interface 24 has been illustrated in aparticular location in the opening 22, it is to be appreciated that themembrane may form at any of various locations within an opening.

As long as the conditions remain relatively constant, the gas columns inthe cells 18 act as an insulative layer which will insulate thecontained liquid from the container walls. If the tanks aredepressurized after the stable membranes 24 have been established, gaswill bubble from the cells 18 to establish a new state of pressureequilibrium. On the other hand, if there is a pressure increase in thetank, additional liquid will flow into the cells 18 and vaporize therebyincreasing the pressure in the gas columns until a new state of pressureequilibrium has been established.

In addition to pressure equilibrium, it is apparent that a thermodynamicequilibrium also must exist across either side of the membrane; althoughthe pressures on either side of the membrane have been described asbeing equalized, the pressure in the gas may be slightly greater thanthe pressure in the adjacent liquid because of capillary pressure forcesand it is important that the liquid immediately adjacent the membrane beat the saturation temperature at this pressure. In other words, it isapparent that the boiling temperature of the liquid, under the pressureconditions present, must exist at the interface.

When a subcooled liquid, that is, one that is at a temperaturesubstantially below its boiling temperature at its local pressure, is incontact with the cover 20, maintaining the liquid at the interface atthe boiling temperature is difficult to achieve. If the liquid at theinterface is below the boiling temperature thereof, condensation of thegas or vapor in the cells will begin to occur. As the vapor condenses,the pressure in the cells will drop and the stable interface isdestroyed. As a result, liquid enters the cells 18 with a resultantflooding thereof. As the liquid enters the cells, some of the liquid mayvaporize thereby increasing the gas pressure in the cells and causingsome of the gas to bubble back into the stored liquid. A percolatingaction will occur and continue as long as the liquid in the vicinity ofthe cover 20 is subcooled.

To eliminate this percolating action and to enable the use of thecapillary insulation concepts in the storage of subcooled liquids, it iscontemplated by the principles of this invention that a thermal barriermay be interposed between the capillary cover 20 and the bulk liquid.This thermal barrier may take the form of a layer of conventionalinsulating material which is placed over the capillary cover 20. Thus,as illustrated in each of FIGS. 1 through 3, a thermal barrier layer,indicated generally by the reference numeral 30, extends across thecellular core 14 and between the capillary cover 20 and the liquid beingstored. This thermal barrier or insulation layer may be of any suitableconventional insulation material, such as cork or a foam material. Thefunction of this insulation layer is to insulate, but not isolate, thegas-liquid interface 24 from the bulk liquid. Hence, the layer 30 needonly be secured to the insulation assembly 10 in such a manner as willmaintain it in position over the cover 20. The layer should not besealed to the cover and, in fact, it is preferred that a relativelyporous material be employed as the insulation layer so that the liquidwill pass through the insulation layer to establish the desired stablecapillary gas-liquid interface in the openings in the cover. If thematerial selected for the insulation layer is not inherently porous,apertures or openings through the material should be provided to permitthe liquid to pass therethrough. By permitting free passage of theliquid through the insulation layer, pressure differences across thislayer are prevented and, therefore, no structural loading is developedwithin the insulation system.

The additional insulation layer 30 functions to insulate the liquidadjacent the membrane from the bulk liquid. If a porous insulationmaterial is used for the additional insulation layer, the porousinsulation material provides a stagnant layer of liquid adjacent thecover 20. By stagnant" is meant that the insulation prevents freeconvection currents to be established in the liquid. The stagnant liquidprovides thermal resistance adequate to establish the necessarytemperature difference.

As shown in FIG. 2, the insulation layer 30 is illustrated as being indirect engagement with the capillary cover 20 with a small space 32between the concave portions of the cover and layer 30. However, thelayer 30 may also be spaced a small distance from the capillary cover 20if so desired.

The composition of the insulation layer, as well as its thickness, mayvary depending on many factors including the minimum heat flux at whichthe system is to be operated and the degree of subcooling of the storedliquid. In general, the layer of insulation must be sufficient to createa temperature stratification with the minimum operating heat fluxthrough the insulation system which, as measured across the insulationlayer, would correspond to a temperature differential equal to thedifference between the boiling and bulk temperatures of the liquid.Thus, referring specifically to FIGS. 2 and 3, the temperature of thebulk liquid L remote from the interface is at the subcooled temperature,while the temperature of the liquid L, adjacent the interface 24 is atthe boiling or saturation temperature. This temperature differential ismaintained primarily by the insulation layer 30 and by the inherenttransfer of some heat inward from the walls of the container. However,in designing a system for subcooled liquids, the insulation layer 30must be so designed that at the minimum permissible heat flux for thesystem, the temperature differential across the insulation layer is suchas to maintain the liquid L, at the boiling temperature.

If the heat transfer to the liquid L, increases above the minimum designlevel, some boiling of the liquid in the space 32 between the insulationlayer 30 and the capillary cover 20 may occur. However, this boilingdoes not adversely affect the insulative function of the insulationassembly, since it will merely cause the liquid-gas interface 24 tointermittently move away from the capillary openings 22. However, therewill be no liquid entry through the openings 22 into the cells 18. Whenthe temperature in this region subsequently returns to the boilingtemperature level, the liquid will return to the region between theinsulation layer 30 and the capillary cover and the stable capillaryinterface will immediately form again in the capillary openings 22.

A primary use of the insulation assembly with the layer is in arelatively large container, where the pressure due to the hydrostatichead at the bottom of the container will be substantially greater thanthe pressure at, for example, a point in the upper region of thecontainer. This differential in pressure will result in a differentialin the boiling temperature of the liquid at the bottom of the containeras compared to the boiling temperature of the liquid at the top of thecontainer. Since the hydrostatic pressure increases with depth in thetank and, consequently, the boiling temperature increases with depth,the degree of subcooling of the bulk liquid increases with depth in thetank. Accordingly, it may be desirable to utilize the additional layerat the bottom of the container but dispense with the insulation layernear the upper portion of the container. Alternatively, the insulationlayer may be used throughout the container but it may be varied inthickness, being thicker at the bottom portion of the container than atthe top. Still further, different insulative materials may be used toform the thermal barrier along different portions of the capillaryinsulation assembly. Thus, one insulative material might be used at thebottom of the container, while a different material might be used at thetop of the container.

While the embodiment described above solves the problem of storingsubcooled liquids by providing a thermal barrier between the vapor inthe cell and the subcooled liquid, it is also possible to solve theproblem created by the subcooled liquid by controlling the vaporpressure within the cell. As noted above, by controlling the pressure ofthe vapor in the cell, it is also possible to control the boilingtemperature of the liquid. If the cell is partially filled with a gasthat is not the vapor of the stored liquid, such as helium, then thesaturation pressure of the stored liquid at the interface corresponds tothe partial pressure of the vapor phase of the contained liquid existingwithin the cell. Accordingly, this, in effect, reduces the pressure ofthe vapor within the cell and thereby results in a reduction in theboiling temperature of the liquid. By properly controlling the amount ofhelium, the partial pressure of the vapor can be controlled within thecell and, ac cordingly, the problem of maintaining a stable interfacewith a subcooled liquid can be controlled. However, over a long periodof time, it might be expected that the helium (or other gas) may beabsorbed into the liquid or displaced by transient bubbling andrefilling of the cell due to pressure or level fluctuation. Accordingly,this is a solution to the problem created by the subcooled liquid inthose environments where storage is for a short period of time.

While the insulation assembly has been disclosed in connection withpreferred embodiments, neither the disclosed embodiments nor theterminology employed in the description thereof is to be limiting;rather, it is intended that the invention be limited only by the scopeof the appended claims.

Having thus described my invention, 1 claim:

1. An insulation assembly for reducing heat transfer between a surfaceand a liquid comprising,

means for providing a layer of gas between the liquid and the surfacewith a stable liquid-gas interface between said gas layer and theliquid, said means including a plurality of cells having a cover closingthe liquid side thereof and having an opening in the cover in which thestable interface is formed, and

means associated with said cover for controlling the thermodynamicrelationship between the liquid at each stable liquid-gas interface andthe vapor of the contained liquid within the respective cells to ensurethat the liquid at each stable liquid-gas interface is not below itsboiling temperature to thereby control the formation of said interface.2. The insulation assembly as defined in claim 1 wherein said meansassociated with said cover for controlling the thermodynamicrelationship comprises means for thermally insulating the interface fromthe liquid remote to the interface.

3. The insulation assembly of claim 2 wherein said means for insulatingsaid interface comprises a thermal barrier operative to establish adifferential between the temperature of the liquid remote from saidinterface and the temperature at the interface.

4. The insulation assembly of claim 3 wherein said temperaturedifferential corresponds to the difference between the boilingtemperature of the liquid at the interface and the temperature of theliquid remote from said interface.

5. The insulation assembly of claim 4 wherein said thermal barriercomprises a layer of material having low thermal conductivity interposedbetween said interface and the bulk of said liquid,

said layer of material being so constructed that the liquid may passfrom one side of said material remote from said interface through thematerial to the other side thereof adjacent to said interface.

6. The insulation assembly of claim 3 wherein said thermal barriercomprises a layer of porous material adapted to receive and store liquidin a relatively stagnant layer to reduce free convection within saidlayer of material.

"I. The insulation assembly of claim 5 wherein said layer of material ispositioned over said cover.

8. The insulation assembly of claim 1 wherein said means for controllingthe thermodynamic relationship includes a gas in said cells other thanthe vapor of the liquid.

9. In an insulation assembly for use in insulating the surface of acontainer for storing a low boiling point liquid therein, and whichassembly includes means defining a plurality of cells for containingrespective columns of gas between the liquid to be stored and thesurface of said container, and a capillary means for said cells forestablishing a capillary interface between each gas column and theliquid, the improvement comprising means for establishing athermodynamic relationship between the liquid at each capillaryinterface and the vapor of the contained liquid within the cell toensure that the liquid at each capillary interface is not below itsboiling temperature.

10. The assembly of claim 9 wherein said means for maintaining thetemperature comprises a thermal barrier positioned between saidinterface and the bulk of said liquid.

11. The assembly of claim 9 wherein said means for maintaining thetemperature is operative to maintain said boiling temperature when thetemperature of the liquid remote from said interface is lower than saidboiling temperature.

12. The assembly of claim 9 wherein said means for establishing thetemperature of the interface comprises a gas in the cells other than thevapor of the liquid so that the pressure of the vapor of the liquid inthe cells comprises a partial pressure of the total pressure in thecells.

1. An insulation assembly for reducing heat transfer between a surfaceand a liquid comprising, means for providing a layer of gas between theliquid and the surface with a stable liquid-gas interface between saidgas layer and the liquid, said means including a plurality of cellshaving a cover closing the liquid side thereof and having an opening inthe cover in which the stable interface is formed, and means associatedwith said cover for controlling the thermodynamic relationship betweenthe liquid at each stable liquid-gas interface and the vapor of thecontained liquid within the respective cells to ensure that the liquidat each stable liquid-gas interface is not below its boiling temperatureto thereby control the formation of said interface.
 2. The insulationassembly as defined in claim 1 wherein said means associated with saidcover for controlling the thermodynamic relationship comprises means forthermally insulating the interface from the liquid remote to theinterface.
 3. The insulation assembly of claim 2 wherein said means forinsulating said interface comprises a thermal barrier operative toestablish a differential between the temperature of the liquid remotefrom said interface and the temperature at the interface.
 4. Theinsulation assembly of claim 3 wherein said temperature differentialcorresponds to the differencE between the boiling temperature of theliquid at the interface and the temperature of the liquid remote fromsaid interface.
 5. The insulation assembly of claim 4 wherein saidthermal barrier comprises a layer of material having low thermalconductivity interposed between said interface and the bulk of saidliquid, said layer of material being so constructed that the liquid maypass from one side of said material remote from said interface throughthe material to the other side thereof adjacent to said interface. 6.The insulation assembly of claim 3 wherein said thermal barriercomprises a layer of porous material adapted to receive and store liquidin a relatively stagnant layer to reduce free convection within saidlayer of material.
 7. The insulation assembly of claim 5 wherein saidlayer of material is positioned over said cover.
 8. The insulationassembly of claim 1 wherein said means for controlling the thermodynamicrelationship includes a gas in said cells other than the vapor of theliquid.
 9. In an insulation assembly for use in insulating the surfaceof a container for storing a low boiling point liquid therein, and whichassembly includes means defining a plurality of cells for containingrespective columns of gas between the liquid to be stored and thesurface of said container, and a capillary means for said cells forestablishing a capillary interface between each gas column and theliquid, the improvement comprising means for establishing athermodynamic relationship between the liquid at each capillaryinterface and the vapor of the contained liquid within the cell toensure that the liquid at each capillary interface is not below itsboiling temperature.
 10. The assembly of claim 9 wherein said means formaintaining the temperature comprises a thermal barrier positionedbetween said interface and the bulk of said liquid.
 11. The assembly ofclaim 9 wherein said means for maintaining the temperature is operativeto maintain said boiling temperature when the temperature of the liquidremote from said interface is lower than said boiling temperature. 12.The assembly of claim 9 wherein said means for establishing thetemperature of the interface comprises a gas in the cells other than thevapor of the liquid so that the pressure of the vapor of the liquid inthe cells comprises a partial pressure of the total pressure in thecells.